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…exactly 3 bytes per code unit StringImpl::tryGetUTF8ForCharacters() grows its first buffer with Vector::resize() when the strict conversion fails. resize() asks for 1.5x the old capacity. For a first buffer of 1,431,655,766 bytes or more that is not a valid capacity, and the process aborts, although the size the caller asked for (3 bytes per code unit) passed the capacity check. The growth is from https://bugs.webkit.org/show_bug.cgi?id=304916 (305091@main) and is in upstream main. The new helper utf8ForIllFormedCharacters() replaces utf8ForCharactersIntoBuffer(). It frees the first buffer, allocates exactly 3 bytes per code unit and reports OutOfMemory when that fails. It reuses the first buffer when it is large enough. StrictConversion returns Invalid with no second buffer. The other modes make one replacing pass (simdutf::convert_utf16le_to_utf8_with_replacement) and do not repeat the strict conversion.
…rogate TestWebKitAPI: WTF.StringUTF8ConversionUnpairedSurrogateAroundBufferSizes converts 16-bit strings with one surrogate at the start, in the middle and at the end, of lengths on both sides of 341 and 1,024 code units, in the three conversion modes and through both forms of the conversion. It compares the result with an encoder written out in the test. JSTests/stress: Date.parse() of a 16-bit string with an unpaired surrogate and a UTF-8 form of 1,431,655,768 bytes. It needs 6 GB, so it is skipped under --memory-limited.
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Status: reproduced on Bun main The Bun pull request that pins the preview build of this branch is oven-sh/bun#44949. |
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Preview build of 15121ff: |
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Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.
Beyond the inline nit, I also checked the new fallback in StringImpl::utf8ForIllFormedCharacters for behavior differences from the upstream path: returning Invalid straight away for StrictConversion matches upstream, since tryConvertUTF16ToUTF8 only reports notFound for unpaired surrogates and Unicode::convert fails on the same input; the clear() before tryReserveCapacity() drops no observable data, because the callback is only invoked after a successful conversion; and a first buffer already at or above 3 bytes per code unit (arm64, trailing surrogate) is reused via an in-place resize() with no reallocation.
Extended reasoning...
The change adds a fork-only (USE(BUN_JSC_ADDITIONS)) replacement for the ill-formed UTF-16 to UTF-8 fallback in Source/WTF/wtf/text/StringImpl.h and StringImpl.cpp, replacing Vector::resize() growth with an exact-size tryReserveCapacity() and a single simdutf replacement pass, plus a TestWebKitAPI test and a 6 GB JSTests stress test. It touches no auth, injection or data-exposure surface. The C++ is 36 lines and reads as equivalent to upstream in bytes and error codes, but it diverges from upstream in a core WTF string path used by every String::utf8() caller, and neither new test runs in fork CI, so a human look is still worthwhile.
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No actionable comments were generated in the recent review. 🎉 ℹ️ Recent review info
WalkthroughThe Bun-specific UTF-16-to-UTF-8 conversion path now handles ill-formed input. New tests check conversion output, strict-mode errors, and ChangesIll-formed UTF-16 conversion
Priority: ⬆️ High Merge Risk: ⚪ Minimal · up to The change targets Date.parse handling of large ill-formed strings. Whether PR CI runs the new regression test remains unknown, but no concrete merge-blocking failure was verified. Pre-merge checks |
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Check skipped because no linked issues were found for this pull request. |
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Check skipped because no linked issues were found for this pull request. |
| Title check | |
The title clearly identifies the main change: allocating an exact-size UTF-8 fallback buffer for ill-formed UTF-16. |
| Description check | |
The description is detailed and covers the problem, fix, testing, performance impact, limitations, and related Bugzilla issue. It does not follow the template exactly because it omits an explicit Revi… |
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Actionable comments posted: 1
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Inline comments:
Review comments at @Source/WTF/wtf/text/StringImpl.cpp:
- Around line 1658-1664: Replace bufferVector.resize(bufferSize) with the Vector
API for setting an uninitialized size, if available, so simdutf can write
directly without zero-filling the buffer. Preserve the existing capacity check,
allocation-failure handling, and conversion call.
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JSTests/stress/date-parse-long-string-with-unpaired-surrogate.jsSource/WTF/wtf/text/StringImpl.cppSource/WTF/wtf/text/StringImpl.hTools/TestWebKitAPI/Tests/WTF/WTFString.cpp
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…runs once, and alone Without a run directive the test ran in every JIT configuration, and slow! put all of them at the front of the run list: about 17 processes of 6 GB at once for a local run without --memory-limited. runDefault runs it in one configuration, as the conversion does not depend on the JIT tier. memoryHog! keeps other tests from running beside it, and skips it under --memory-limited as before.
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I reviewed the latest push and found no bugs; since this is a fork-only divergence in a WTF conversion path that every String::utf8() / tryGetUTF8() call on 16-bit strings goes through, a human look at the memory trade-off would still be worthwhile.
What was reviewed:
- Confirmed the crash mechanism in Vector.h:
resize()->expandCapacity()asks for max(size*3, 1.5x capacity) withFailureAction::Crash; the newclear()+tryReserveCapacity()+resize()sequence avoids the 1.5x request becauseresize()does not regrow when capacity already suffices. - Checked the simdutf span overload of
convert_utf16le_to_utf8_with_replacementexists in the vendored header and that 3 bytes per code unit bounds its output; the strict-mode earlyInvalidis sound sinceconvert_utf16le_to_utf8_with_errorsonly fails on surrogates. - The third commit's
//@ runDefault+//@ memoryHog!resolves the earlier scheduling nit;memoryHog!isexclusive!plus skip under--memory-limitedin run-jsc-stress-tests. - One thing for a human to weigh: ill-formed strings that previously converted in a smaller buffer now always get a 3x-size buffer (acknowledged in the description), and there is an open coderabbit inline comment on StringImpl.cpp:1664 that I could not confirm was addressed.
Extended reasoning...
The change replaces the upstream fallback for ill-formed UTF-16 in StringImpl.h/.cpp with a fork-only helper under USE(BUN_JSC_ADDITIONS) that reserves exactly 3 bytes per code unit and runs simdutf's replacing converter once, plus a TestWebKitAPI test and a 6 GB JSTests stress test. It touches no auth, injection, or data-exposure surface; it is a memory-allocation and string-conversion hot path shared by every utf8() call on 16-bit strings. The core logic is small and I verified it against Vector.h's resize/expandCapacity/reserveCapacity and the vendored simdutf overload, and the new commit addresses my prior inline nit. Deferring rather than approving because it is a permanent divergence from upstream in a widely used WTF path with a memory-usage trade-off, and because a coderabbit inline comment on StringImpl.cpp:1664 has no visible resolution and no later commit touched that file.
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Problem
panic(main thread): abort() calledand exits 134, also insidetry/catch.StringImpl::tryGetUTF8ForCharactersregrows its buffer withVector::resize()(StringImpl.h:1537), which asks for 1.5x the capacity. From a first buffer of 1,431,655,766 bytes that is over theVectorlimit, andCRASH()follows (Vector.h:228).Fix
utf8ForIllFormedCharacters()replaces theresize()andutf8ForCharactersIntoBuffer(), underUSE(BUN_JSC_ADDITIONS). It allocates exactly 3 bytes for each code unit, and a failed allocation isOutOfMemory. It makes one replacing pass with simdutf and no second strict pass.WTF.StringUTF8ConversionUnpairedSurrogateAroundBufferSizesand aJSTests/stressfile for the abort.Background
String::utf8(),tryGetUTF8()andStringView::tryGetUTF8()all end in this function. Only an unpaired surrogate makes its strict conversion fail, and then a second conversion writes U+FFFD.Vector::expandCapacityalso stops the abort, but this site would still allocate too much and copy. A converter in Bun does not reachDate.parse.Downsides
UTF8CStringstill crashes.Notes
Repro. Each of these exits 134 in a release build of Bun main (
e655c58032, linux x64):The same string through
StringImpl::utf8ForCharacters()andtryGetUTF8ForCharacters()in a program linked with the pin'slibWTF.a: exit 134 in both forms and in the three modes. With this change: 1,431,655,768 bytes that end ine6 97 a5 ef bf bd 61, peak RSS 3,644 MB (UTF8CStringform) and 2,278 MB (callback form).StrictConversiongivesInvalid. In a release build of Bun with the preview build of this branch (oven-sh/bun#44949) the three calls giveNaN,SQLiteError: statement too longandERR_SQLITE_ERROR: string or blob too big.Why there is a second buffer. simdutf does not define
utf8_length_from_utf16lefor ill-formed input. Its kernels count an unpaired surrogate as 2 bytes in most positions, and U+FFFD takes 3. The arm64 kernel can count 4 for one at the very end. So the first buffer can be too small for the replaced form, and upstream grows it to 3 bytes for each code unit.The strings that abort. The first buffer is 1,431,655,766 bytes or more, so that
capacity + capacity / 2is over 2^31 - 1. The string has at most 715,827,882 code units, so that 3 bytes for each pass the check in front of theresize(). A string under that size converted before, in a buffer of up to 2^31 - 1 bytes. Now its buffer is 3 bytes for each code unit.What the helper keeps from upstream. The order of the checks:
OutOfMemorywhen 3 bytes for each code unit are not a valid capacity, thenInvalidforStrictConversion. The bytes:simdutf::convert_utf16le_to_utf8_with_replacementwrites whatUnicode::convertReplacingInvalidSequences()writes. It does not repeatconvert_utf16le_to_utf8_with_errors, which fails again for the same string. A first buffer that is already large enough is used again.Instructions and allocations for one call.
StringImpl.cpp,CString.cppandURL.cppcompiled frommainand from this branch with the command lines that the pin's prebuilt recorded, and linked in front of the pin'slibWTF.a. A tracer single-steps the call and counts. The allocator behindfastMallocis a stub with a fixed cost that counts calls, andmemcpycosts one instruction for each byte. 200 inputs (Latin-1 and UTF-16, well-formed and with an unpaired surrogate, 0 to 65,536 units) in the callback form and in theUTF8CStringform, with the simdutf kernel set to icelake, haswell and westmere, and on arm64 under qemu-user.UTF8CStringform unless noted)The rows below are the strings with an unpaired surrogate, and well-formed UTF-16 by 1 to 3 instructions.
Same bytes. A generator gives both builds the same inputs: every string of up to 6 units over 6 kinds of code unit, one or two surrogates at every position of strings around 341 and 1,024 units, and random mixtures up to 69,000 units. Each input goes through both forms in the three modes. 15,032,260 comparisons of the base build with this one on the icelake, haswell, westmere and arm64 kernels: 0 differences. Each build also compares every result with
Unicode::convertReplacingInvalidSequences()andUnicode::convert().Tests and CI. Fork CI runs neither test: it passes
--memory-limited(the stress file needs 6 GB) and does not build TestWebKitAPI. The stress file exits 134 on the pin'sjsc. The TestWebKitAPI test passes with and without this change: it holds the bytes and the errors of the second conversion, which is a different converter now.TestWTFbuilt locally: the new test passes, and 281 other String, URL and UTF tests pass. Two things are local to that build: theExternalStringImpltests inTests/WTF/StringImpl.cppdo not compile in the fork today and were disabled, andWTF.StringViewIteratorswas left out because ICU could not open a break iterator in that build directory. oven-sh/bun#44949 has the tests that CI runs.Upstream merges.
git merge-fileof this patch with the reverse of upstream changes to these lines: the merge of 2026-10-06 conflicts in 0 hunks of the two source files and in 1 hunk of the test file. Thegrowtoresizechange and the simdutf span change conflict in 0. The change that added these lines (bug 304916) conflicts in 2.Self-review. The first version of this branch also rewrote both conversion functions for speed and made every allocation of the conversion fallible (270 more lines). The review asked for five things:
The speed work stays on the branch
robobun/99d68896/utf8-conversion-fallible. It needs wall-clock numbers for both encodings before it is a pull request.Not in this PR.
UTF8CStringstill come from allocations that crash when they fail.String::utf8(),StringView::utf8()andStringImpl::utf8()stillRELEASE_ASSERTthat the conversion worked.Vector::expandCapacitystill asks for 1.5x for every otherVectorthat grows near its limit. [WTF] Vector: limit the capacity by the element count, not by half the size in bytes (regression from 310668@main) #691 is about the limit itself.